The evaluation of jugular venous pulse has been an integral part of cardiovascular examination and has important clinical diagnostic values [1-2]. Jugular venous pulse is produced by the changes in blood flow and pressure in central veins caused by right atrial and ventricular filling and contraction. The two main objectives of the bedside examination of jugular vein pulse include the estimation of central venous pressure and the inspection of the waveform. Because of its more direct route to the right atrium, the right internal jugular vein is superior for the purpose. Based upon these measurements, physicians can access hemodynamic events in the right atrium and thus diagnose heart diseases and abnormalities. For example, the most common cause of elevated jugular venous pressure is an increase in right ventricular pressure such as occurs in patients with pulmonary stenosis, pulmonary hypertension, or right ventricular failure secondary to right ventricular infarction. The venous pressure also is elevated when obstruction to right ventricular inflow occurs, such as with tricuspid stenosis or right atrial myxoma, or when constructive pericardial disease impedes right ventricular inflow. It may also result from vena caval obstruction and, at times, an increased blood volume. Patients with obstructive pulmonary disease may have an elevated venous pressure only during expiration.
The conventional technique for measuring venous pulse and waveform has been described in the literature [3]. The patient is examined at the optimum degree of trunk elevation for visualization of venous pulsations. The venous pressure is measured by a ruler as the vertical distance from the top of the oscillating venous column, to the level of the sternal angle plus vertical distance to the right atrium. Due to the fact that the venous pulse is in generally very small, and due to complications with patients, this method is challenging or physicians to use and provides approximate values only.
Cardiac output is defined as the volume of blood circulated per minute. It is equal to the heart rate multiplied by the stroke volume (the amount ejected by the heart with each contraction). Cardiac output is of central importance in the monitoring of cardiovascular health [4]. Accurate clinical assessment of circulatory status is particularly desirable in critically ill patients in the ICU and patients undergoing cardiac, thoracic, or vascular interventions, and has proven valuable in long term follow-up of outpatient therapies. As a patient's hemodynamic status may change rapidly, continuous monitoring of cardiac output will provide information that allows rapid adjustment of therapy. Measurements of cardiac output and blood pressure can also be used to calculate peripheral resistance.
Jansen (J. R. C. Jansen, “Novel methods of invasive/non-invasive cardiac output monitoring”, Abstracts of the 7th annual meeting of the European Society for Intravenous Anesthesia, Lisbon 2004) describes eight desirable characteristics for cardiac output monitoring techniques; accuracy, reproducibility or precision, fast response time, operator independency, ease of use, continuous use, cost effectiveness, and no increased mortality and morbidity.
Pulmonary artery catheter (PAC) thermodilution method is generally accepted as the clinical standard for monitoring cardiac output, to which all other methods are compared as discussed by Conway and Lund-Johansen [6]. As this technology is highly invasive, complicated, and expensive, many new methods have been developed in an attempt to replace it, but none have so far gained acceptance. A recent review of the various techniques for measuring cardiac output is given in Linton and Gilon [5]. This article lists both non/minimally invasive and invasive methods and compares the advantages and disadvantages of each. A brief description of some of these techniques follows.
Indicator dilution techniques. There are several indicator dilution techniques including transpulmonary thermodilution (also known as PiCCO technology, Pulsion Medical Technologies of Munich, Germany), transpulmonary lithium dilution method (LiDCO Group plc or London, UK), PAC based thermo-dilution and other methods (Vigilance, Baxter; Opti-Q, Abbott; and TruCCOMS, AorTech). Application of such techniques assumes three major conditions, namely, complete mixing of blood and indicator, no loss of indicator between place of injection and place of detection, and constant blood flow. The errors associated with indicator dilution techniques are primarily related to the violation of these conditions, as discussed by Lund-Johansen [7-8].
Fick principle. The direct oxygen Fick approach is currently the standard reference technique for cardiac output measurement as discussed by Keinanen et al [9-10]. It is generally considered the most accurate method currently available. The NICO (Novametrix) system is a non-invasive device that applies Fick's principle and relies solely on airway gas measurement as described by Botero et al [11]. This method shows a lack of agreement between thermodilution and CO2-rebreathing cardiac output as described in Nielsson et al [12], due to unknown ventilation/perfusion inequality in patients.
Bio-Impedance and conduction techniques. The bio-impedance method was developed as a simple, low-cost method that gives information about the cardiovascular system and/or (de)-hydration status of the body in a non-invasive way. Over the years, a diversity of thoracic impedance measurement systems have also been developed. These systems determine CO on a beat-to-beat time basis. Studies have been reported with mostly poor results, but in some exceptional cases, there was good correlation with a reference method. Many of these studies refer to the poor physical principles of the thoracic impedance method as described in Patterson “Fundamentals of impedance cardiography”, IEEE Engineering in Medicine and Biology 1989; 35 to explain the discrepancies.
Echo-Doppler ultrasound. This technique uses ultrasound and the Doppler Effect to measure cardiac output. The blood velocity through the aorta causes a ‘Doppler shift’ in the frequency of the returning ultrasound waves. Echo-Doppler probes positioned inside the esophagus with their echo window on the thoracic aorta may be used for measuring aortic flow velocity, as discussed by Schmidlin et al [13]. Aortic cross sectional area is assumed in devices such as the CardioQ, made by Deltex Medical PLC, Chichester, UK, or measured simultaneously as, for example, in the HemoSonic device made by Arrow International. With these minimally invasive techniques what is measured is aortic blood flow, not cardiac output. A fixed relationship between aortic blood flow and cardiac output is assumed. Echo-Doppler ultrasound requires an above average level of skill on the part of the operator of the ultrasound machine to get accurate reliable results.
Arterial pulse contour analysis. The estimation of cardiac output based on pulse contour analysis is an indirect method, since cardiac output is not measured directly but is computed from a pressure pulsation on the basis of a criterion or model [14-17]. Three pulse contour methods are currently available; PiCCO (Pulsion), PulseCO (LiDCO) and Modelflow (TNO/BMI). All three of these pulse contour methods use an invasively measured arterial blood pressure and they need to be calibrated. PiCCO is calibrated by transpulmonary thermodilution, LiDCO by transpulmonary lithium dilution and Modelflow by the mean of 3 or 4 conventional thermodilution measurements equally spread over the ventilatory cycle.
Near infrared spectroscopy has been used to non-invasively measure various physiological properties in animal and human subjects. The basic principle underlying near infrared spectroscopy is that a physiological medium such as tissues includes a variety of light-absorbing (chromophores) and light-scattering substances which can interact with transmitted low energy near infrared photons. For example, deoxygenated and oxygenated hemoglobins in human blood are the most dominant chromophores in the spectrum range of 400 nm to 1000 nm. Therefore, diffuse optical spectroscopy has been applied to non-invasively measure oxygen levels in the physiological medium in terms of tissue hemoglobin oxygen saturation. Technical background for diffuse optical spectroscopy has been discussed in, e.g., Neuman, M. R., Pulse Oximetry: Physical Principles, Technical Realization and Present Limitations.@ Adv. Exp. Med. Biol., vol. 220, p. 135-144, 1987 and Severinghaus, J. W., History and Recent Developments in Pulse Oximetry,@ Scan. J. Clin. and Lab. Investigations, vol. 53, p. 105-111, 1993.
Because of the highly scattering nature of tissue to the visible and near infrared light (400 nm-1000 nm), it is difficult to apply diffuse optical spectroscopy non-invasively to select blood vessels within a tissue to calculate blood oxygenation. Thus, diffuse optical spectroscopy has only been used to measure the combined or average oxygenation of blood from arteries, veins, and capillaries within a tissue medium. However, in many clinical applications, it is desirable to know the blood oxygenation of particular blood vessels. To do so, various invasive methods have been developed which involve the use of catheters that must be inserted into a targeted blood vessel to make the measurement.
None of the above-mentioned techniques of measuring cardiac output combines all of the eight “Jansen” criteria mentioned above and, thus, none can displace the conventional thermodilution technique as described by Jansen et al [18]. Although highly invasive, complicated and expensive, the conventional thermodilution method remains the method of choice for measuring cardiac output. Given the foregoing, it would be highly desirable to develop a non-invasive method for real-time monitoring of cardiac output in a clinical setting which is accurate, reliable, cost effective and easy to use.